1. Field of the Invention
The present invention relates to a thermal print head, and more particularly to a thermal head having an improved glazed layer for enhancing printing efficiency and also to a printing device equipped with the same.
2. Description of the Related Art
A thermal print head generates heat in response to supplied driving current to perform printing on a heat sensitive sheet. In this case, it is general to provide a glazed layer beneath a heat generating resistor as a heat accumulating layer, due to a large heat dissipation capacity of a ceramic substrate.
The glazed layer acts to prevent the heat from dissipating so as to improve the energy efficiency. Without this glazed layer, an excessively large amount of energy would be required to sufficiently heat the head in order to start printing, resulting in significantly poor energy efficiency. If the glazed layer is too large, however, the heat dissipation capacity would become deteriorated so as to prevent the once heated printing section (heat generating section) from quickly cooling. As a result, this residual heat causes a tailing phenomenon.
In this manner, while the glazed layer in a thermal head acts as a heat accumulating layer to improve the energy efficiency, it may also lead to lowering of the printing quality. The shape and size of the glazed layer play important roles and should be determined in view of the relationship to the energy efficiency and the printing quality.
It has been impossible, however, to perform the printing operation at a speed higher than approximately 3 msec which is the cooling time of the conventional print head. For increasing the printing speed, so-called heat history control has been used, in which the heat generating amount is varied by adjusting the printing energy in view of printing data stored in a memory. Even with this control, however, the critical recording speed is approximately 0.8 msec, at which speed it is not easy to carry out printing with high quality.
Thus, in the conventional printing device provided with such a type of thermal print head, the printing quality was not sufficient in the case of high speed printing. High speed printing of bar codes, for example, would easily result in insufficient printing of bars, leading to reading errors.